Unsaturated polyester (UP) resins are one of the most widely used thermoset polymers. Their major applications are composite products manufactured by compression molding in the form of sheet molding compounds (SMC) or bulk molding compounds (BMC), injection molding in the form of BMC, resin transfer molding (RTM), casting, and hand lay-up. Because of their light weight, high strength and non-corrosive nature, unsaturated polyester resins have replaced sheet metal in many applications, particularly in the automotive, electric and home appliance industries.
Unsaturated polyester resins are typically made by reacting an unsaturated dicarboxylic acid or anhydride, such as maleic anhydride or fumaric acid, with a polyol such as propylene glycol to form a low molecular weight unsaturated polyester resin (LMWUPR). For SMC and BMC applications, such LMWUPR's are generally thickened for easy handling and good fiber carrying characteristics during mold filling.
Chemically, thickening or "maturation" occurs by linking various LMWUPR molecules together to form polymer chains of considerably higher molecular weight. Typically, this is done by adding to the system a di- or multi-functional compound which couples two or more polyester molecules together via their terminal hydroxyl and/or carboxyl groups. Because, the LMWUPR molecules usually contain more than two functional groups, the actual product formed is more typically a complex network of interconnected polymer chains rather than discrete individual chains.
Compounds used for thickening LMWUPR's are known in the art by various terms such as "thickness", "thickening agents" and "maturation agents." Essentially two types of compounds are used for this purpose. One type comprises Group IIA metal oxides and hydroxides. MgO is the most common agent of this type. It is now well accepted that maturation with this type of agent occurs via formation of ionic bonds through the reaction of MgO or analogue with the carboxylic acid end groups of the polyester molecule.
The other type of maturation agent is diisocyanate. Diisocyanates operate by forming covalent bonds, specifically urethane linkages, with the terminal hydroxyl groups of the polyester molecule.
Each type of maturation agent has its own advantages and disadvantages. For example, it is desirable that viscosity increase occur very rapidly during maturation and further that viscosity remain stable for extended periods of time once maturation is completed. Diisocyanate maturation agents exhibit this desirable property, but MgO-type maturation agents do not. Moreover, MgO-type maturation agents are very sensitive to humidity after maturation, whereas diisocyanates are not.
On the other hand, it is desirable during molding that the UPR resin exhibit good material flow. This facilitates complete filling of the mold as well as complete wetting of the filler and other ingredients in the system by the UPR. The ionic bonds formed when MgO-type maturation agents are used weaken at the elevated temperatures encountered in molding. This results in reduced compound viscosity and hence the desired material flow. The covalent bonds formed with isocyanate type thickeners, however, do not weaken at molding temperatures and hence material flow is more difficult.
It is well known that molded articles made with conventional UPR resins often exhibit poor surface finish. This is believed due to the fact that UPR's shrink somewhat as a result of the molding operation. To deal with this problem, it is also well known to add to the system (i.e, the total composition including both the UPR and all other ingredients) certain ingredients known as low profile agents (LPA). Although LPA's are effective, good material flow during molding is still necessary to obtain finishes of the highest quality. The reduced material flow encountered when diisocyanates are used as thickeners reduces LPA effectiveness in these systems, which in turn may lead to significant finish problems.
Attempts have been made to develop UPR systems whose viscosity profile exhibits all of the above beneficial properties, namely rapid increase during maturation, long term stability and significant viscosity reduction during molding. For example, one proposal has been to use both MgO type and diisocyanate type thickeners in the same system. See Melby, E. G. and Castro, J. M., 7, Ch. 3 Comprehensive Polymer Science, Pergamon Press, Oxford, UK (1989), the disclosure of which is incorporated herein by reference. To date, however, such systems have not been found effective as a practical matter.
Accordingly, it is an object of the present invention to provide a new UPR system which exhibits the rapid and stable thickening properties of a diisocyanate-thickened UPR system but which, at the same time, exhibits the reduction in viscosity and hence good material flow at molding conditions exhibited by UPR systems thickened with MgO type maturation agents.
In addition, it is another object of the present invention to provide novel components of UPR systems, in particular novel modified low molecular weight unsaturated polyesters and novel low profile additives, which can be used either individually or together to form the above UPR systems.
In addition, it is a still further object of the present invention to provide a novel technique for formulating specific UPR systems which allows the desired viscosity profile to be imparted to the system, even though the specific components thereof vary widely.